In recent years, MIMO (Multi-Input/Multi-Output) communication attracts attention as a technique that enables communication of large amount of data such as image. In MIMO communication, different transmission data (substreams) is respectively transmitted from a plurality of antennas of a transmitting side, and, on the receiving side, a plurality of transmission data mixed together on a channel is demultiplexed to the original transmission data using a channel estimation value.
In actuality, in MIMO communication, signals transmitted from a transmitting apparatus are received at the number of antennas larger than or equal to the number of transmitting apparatuses, and the channel characteristics between the antennas are estimated based on pilot signals inserted in the signals received at the antennas.
The estimated channel characteristics H is expressed by the matrix 2×2 when there are two transmitting antennas and two receiving antennas, for example. In MIMO communication, based on the inverse matrix of the obtained channel characteristics H and the received signals obtained at receiving antennas, the transmission signals (substream) transmitted from transmitting antennas are obtained.
The principle of MIMO communication, when the number of antennas of transmitter 10 and receiver 20 is two, respectively, will be described using FIG. 1A. Here, the signals transmitted from antennas 11 and 12 of transmitter 10 are TX1 and TX2, respectively, and the signals received by antennas 21 and 22 of receiver 20 are RX1 and RX2, respectively. At this time, the received signals (RX1 and RX2) can be expressed by equation 1 shown in FIG. 1B.
Here, in equation 1, A indicates the channel characteristics between transmitting antenna 11 and receiving antenna 21, B indicates the channel characteristics between transmitting antenna 12 and receiving antenna 21, C indicates the channel characteristics between transmitting antenna 11 and receiving antenna 22 and D indicates the channel characteristics between transmitting antenna 12 and receiving antenna 22.
At this time, when only signal TX1 is transmitted to receiver 20, for example, TX2 becomes an interference signal for receiver 20, and the signal received by antenna 21 includes both the desired signal component and interference signal component. The same holds true for antenna 22.
In order to remove (compensate) the above interference signal component from the received signals and obtain the transmission signals (TX1 and TX2), it is necessary to obtain the inverse matrix of the matrix formed with four channel characteristics A, B, C and D, as shown in equation 2. Therefore, transmitter 10 transmits a signal where a known signal (for example, a pilot signal) for channel estimation is inserted in the transmission signal, and receiver 20 performs channel estimation based on this known signal, obtains channel characteristics A, B, C and D, and obtains the above inverse matrix.
In actuality, the steps for obtaining the transmission signals (TX1 and TX2) from the received signals (RX1 and RX2) include operations such as a ZF (Zero-Forcing) operation that demultiplexes substreams (each data) only through the inverse matrix operation expressed by equation 2 or an MMSE (Minimum Mean Square Error) operation that performs demultiplexing so as to minimize an error.
Thus, in MIMO communication, theoretically, a plurality of signals transmitted at the same frequency and at the same time can be respectively demultiplexed at the receiver, thereby enabling high-speed and high-capacity communication.
By the way, in a MIMO communication scheme, a plurality of transmission systems that use radio sections provided with power amplifiers having large power consumption are required on the transmitting side. It is well known that, when a MIMO communication scheme is applied to uplink, the power consumption of receiver 20 will become extremely large. Further, downlink is considered important with respect to the throughput of a MIMO communication scheme. For these reasons, a MIMO communication scheme is generally used only in downlink.
In such a MIMO communication scheme, as disclosed in Non-Patent Document 1, to further improve the throughput, a method is studied of independently setting a transmission rate per antenna and transmitting a CQI (Channel Quality Indicator), which is a transmission rate setting signal of each antenna.
A CQI is a signal that indicates the modulation scheme and coding rate of packet data that can be demodulated in receiver 20. Transmitter 10, such as a base station, for example, has the CQI transmitted from receiver 20 at a period set by an upper apparatus such as an RNC (Radio Network Controller). Transmitter 10 receiving the CQI performs scheduling using the CQI transmitted from receiver 20 and independently selects per antenna the optimum modulation scheme and coding rate. Then, transmitter 10 modulates and encodes the transmission data using the selected modulation scheme and coding rate, and transmits the data to receiver 20 based on the scheduling result. By this means, by adaptively changing the transmission rate according to the radio wave propagation environment, it is possible to transmit large amount of data from transmitter 10 to receiver 20.    Non-Patent Document 1: 3GPP TR25.876